@Lee_Jay wrote:
I have a 7D Mark II, but I understand this is the same with other cameras.
When you set auto ISO in manual mode, and change exposure compensation, it changes ISO. Good.
I think you may find this depends on the camera mode. Generally the camera follows something called the "program line". In auto & semi-auto exposure modes, it tries to find a "safe" exposure (trying to find a reasonably safe shutter speed based on focal length of the lens and trying to find a safe f-stop given the aperture range of the lens.) If the camera cannot open the aperture further AND slowing the shutter speed (relative to the focal length) would be unwise... then the safest thing to do is change the ISO.
But the computer has "options" and, in my experience, which option it chooses depends on how much wiggle room it has with each element of the exposure (ISO, aperture, and shutter speed.) You can read articles about how the "program line" works and realize the logic (what the camera will change is predictable -- and is based on which elements of the "exposure triangle" are already constrained.)
When you set auto ISO in manual mode, and use auto exposure bracketing, it changes shutter speed. Not good.
The general recommendation in the industry when bracketing is that you should alter the shutter speed -- which is why the camera alters the shutter speed.
When you "bracket", the assumption is that you intend to use the bracketed data to merge for purposes of an HDR type result. If you alter the aperture, then you change the depth of field. This would be bad because you want all the samples to have identical depth of field with the ONLY variable being the amount of light collected. If you change the ISO then you change the amount of noise and this may also be undesirable because you get a brighter exposure to integrate into the HDR result... but the "brighter" photo is now also noisier and that degrades the quality of the result.
BUT... if you only change the shutter speed... you're generally safe. The depth of field and noise levels should be consistent across all exposure samples in you bracketed series. This means you may need a tripod. That's probably ok, because bracketing doesn't work well for "action" photography (each image captures something completely different and you want a series of shots that captured the same thing... with only the exposure being different.) This means you PROBABLY should be using a tripod anyway (otherwise you have to "register" (align) the frames because you didn't use a tripod... then crop.)
So... if you're using a tripod (and you should be) then the safest and best option to vary for a bracket image is... the shutter speed. And that's exactly what the camera does.
I understand the thinking here, even if I think it's wrong, so instead of just "fixing it", I'd recommend a custom function for selecting which you want it to do. In my case, I actually want manual mode to behave like manual mode rather than behaving like Av mode which is the current behavior with AEB in manual mode. If I had wanted the behavior to be like Av mode, I would have set it in Av mode!
There's nothing magical about bracketing. Some cameras have a built-in HDR mode which combines the images for you. Some have a built-in "bracketing" mode which simplifies the task of gather a series of images where each has a slightly different exposure value (but doesn't combine them). But it turns out photographers have been bracketing for years before any camera had any built-in feature for "bracketing".
Take a shot, alter your ISO, take another, alter the ISO again, and take another (then reset back to the original ISO). The camera cannot stop you from doing that. If you use the automatic bracketing feaure, it's going to use the shutter speed as the variable because that's the control that most photographers want to use -- and it's the method that would be taught in photography books and photography schools.
Tim Campbell
5D III, 5D IV, 60Da
IMHO, of coures, as always.
